/Engine/Source/Runtime/Media/Public/IMediaMetadataItem.h
Optional mime type of the item described by Value. This should be set when Value is of type EVariantTypes::ByteArray For example, if this item is a binary JPEG image the mime type should be set as "image/jpeg" to identify the type of the payload.
const [FString](API\Runtime\Core\Containers\FString) & GetMimeType() const